In the core book there are a number of references to Magic Resistance rolls. While I have been playing the game for a quite a while now, I have no idea what these are. Here are the examples I can find:
MAGIC SENSITIVITY (Minor Supernatural Virtue, p. 45)
However, your sensitivity makes you more susceptible to magical effects: subtract your Magic Sensitivity score from all magic resistance rolls you make.
LIMITED MAGIC RESISTANCE (Minor Hermetic Virtue, p.55)
You gain no bonus from one of your Form scores to Magic Resistance rolls...
MAGIC SENSITIVITY (Ability, p.66)
subtract your Magic Sensitivity score from all magic resistance rolls you make
CERTAMEN (p. 90)
This spell, which must be of the same Technique and Form as the contest, circumvents the loser’s Parma Magica (though the loser still gets a Magic Resistance roll based on the Form of the spell).
POSING THE SILENT QUESTION (InMe spell, P. 149)
The target of this spell does not notice the questioning unless he or she makes a Magic Resistance roll.
I can only assume that these must be cases of poorly expressing the rules, and what is meant is that Magic Resistance is affected in the ways described and the roll would be the casting roll (or roll of a supernatural ability, etc). And so in the final spell in Certamen a defeated mage would still benefit from their base Magic Resistance from knowledge of the Hermetic form while in the case of Posing the Silent Question, if the spell comes off but fails to penetrate then the target of the spell notices the questioning, but otherwise they do not.
But it is odd that there are these repeated references to Magic Resistance rolls as if an additional roll against Magic Resistance was required in some instances. As it happens I quite like the idea of a Magic Resistance roll to see whether a spell penetrates. I find the rules on MR a bit too cut and dried for my tastes, and for dramatic effect if nothing else I would quite like mundanes with no MR to have a slight chance to resist a spell and likewise a spell cast against a powerful being or Hermetic wizard to have a slight chance to penetrate despite going up against a high MR.